In an electrostatographic copy machine, an electrostatic latent image is formed on an element. That image is developed by the application of an oppositely charged toner to the element. The image-forming toner on the element is then transferred to a receiver where it is permanently fixed, typically by heat fusion. The transfer of the toner to the receiver is usually accomplished electrostatically by means of an electrostatic bias between the receiver and the element.
In order to produce copies of very high resolution and low granularity, it is necessary to use toner particles that have a very small particle size, i.e., less than about 8 micrometers. (Particle size herein refers to mean volume weighted diameter as measured by conventional diameter measuring devices such as a Coulter Multisizer, sold by Coulter, Inc. Mean volume weighted diameter is the sum of the mass of each particle times the diameter of a spherical particle of equal mass and density, divided by total particle mass.) However, it has been found that it is very difficult to electrostatically transfer such fine toner particles from the element to the receiver, especially when they are less than 6 micrometers in diameter. That is, fine toner particles frequently do not transfer from the element with reasonable efficiency. Moreover, those particles which do transfer frequently fail to transfer to a position on the receiver that is directly opposite their position on the element, but rather, under the influence of coulombic forces, tend to scatter, thus lowering the resolution of the transferred image and increasing the grain and mottle. Thus, high resolution images of low granularity require very small particles, however, images having high resolution and low granularity have not been attainable using electrostatically assisted transfer.
In order to avoid this problem, it has become necessary to transfer the toner from the element to the receiver by non-electrostatic processes. One such process is the thermally assisted transfer process where the receiver is heated, typically to about 60.degree. to about 90.degree. C., and is pressed against the toner particles on the element. The heated receiver sinters the toner particles causing then to stick to each other and to the receiver thereby effecting the transfer of the toner from the element to the receiver. The element and receiver are then separated and the toner image is fixed, e.g., thermally fused to the receiver. For details, see copending application Ser. No. 230,394, titled "Thermally Assisted Transfer of Small Electrostatographic Toner Particles" filed Aug. 9, 1988, now U.S. Pat. No. 4,927,727.
While the thermally assisted transfer process does transfer very small particles without the scattering that occurs with electrostatic transfer processes, it is sometimes difficult to transfer all of the toner particles by this process. The toner particles that are directly on the element often experience a greater attractive force to the element than they do to the receiver and to other toner particles that are stacked above them, and the heat from the receiver may have diminished to such an extent by the time it reaches the toner particles next to the element that it does not sinter them. As a result, the toner particles that are in contact with the element may not transfer. Attempts to solve this problem by coating the element with a release agent have not proven to be successful because the process tends to wipe the release agent off the element into the developer which degrades both the developer and the development process. Moreover, because the process tends to wipe the release agent off the element, the application of additional release agent to the element is periodically required in order to prevent the toner particles from adhering to the element during transfer.
An alternative approach to removing all of the toner particles from the element is to use a receiver that has been coated with a thermoplastic polymer. During transfer, the toner particles adhere to or become partially or slightly embedded in the thermoplastic polymer coating and are thereby removed from the element. However, it has been found that many thermoplastics that are capable of removing all of the toner particles also tend to adhere to the element. This, of course, not only seriously impairs image quality but it may also damage both the element and the receiver. Moreover, it was not possible to predict with any degree of certainty which thermoplastic polymers would remove all of the toner particles from the element without sticking to the element during transfer and subsequent separation of the receiver from the element and which ones would not.
In copending U.S. application Ser. No. 345,160, entitled "Method of Non-Electrostatically Transferring Toner" filed Apr. 28, 1989, which is a continuation-in-part in of U.S. application Ser. No. 230,381, entitled "Improved Method of Non-Electrostatically Transferring Toner" filed Aug. 9, 1988, it is disclosed that if such small sized toner particles are transferred to a receiver formed of a substrate or a support which has been coated with a thermoplastic polymer having a layer of a release agent on the thermoplastic polymer coating and the receiver is heated above the Tg of the thermoplastic polymer during transfer, the release agent will prevent the thermoplastic polymer coating from adhering to the element but it will not prevent the toner from transferring to the thermoplastic polymer coating on the receiver and virtually all of the toner will transfer to the receiver. This constitutes a significant advancement in the art because it is now possible not only to obtain the high image quality that was not previously attainable when very small toner particles were transferred electrostatically but, in addition, the problem of incomplete transfer is avoided. In addition, several other advantages are provided by this process. One such advantage is that copies made by this process can be given a more uniform gloss because all of the receiver is coated with a thermoplastic polymer which can be made glossy while, in receivers that are not coated with a thermoplastic polymer, only those portions of the receiver that are covered with toner can be made glossy and the level of gloss varies with the amount of toner. Another advantage of the process is that when the toner is fixed, it is driven more or less intact into the thermoplastic polymer coating rather than being flattened and spread out over the receiver. This also results in a higher resolution image and less grain. Finally, in images made using this process, light tends to reflect from behind the embedded toner particles that are in the thermoplastic layer which causes the light to diffuse more making the image appear less grainy.
For all of the benefits and advantages provided by this process, however, the application of a release agent to the thermoplastic polymer coating on the receiver in order to prevent the thermoplastic polymer coating from adhering to the surface of the element during transfer and subsequent separation of the receiver from the element creates several problems. One such problem is that the release agent tends to transfer to and build up on the element or photoconductor thereby degrading image quality and causing potential damage to both the element and the receiver. Another problem is that the release agent tends to allow the thermoplastic polymer coating to separate from the support or substrate, especially during or after finishing due to a reduction in the adhesion strength of the thermoplastic polymer coating to the receiver support caused by the tendency of the release agent, which has a lower surface energy than the thermoplastic polymer coating and hence a lesser predilection to adhere to the receiver support than the thermoplastic polymer coating, to migrate through the thermoplastic polymer coating to the interfacial region between the thermoplastic polymer coating and the support and to cause the thermoplastic polymer coating to separate from the support. It has also been found that the release agent reduces the gloss of the finished image. Finally, the addition of a release agent to the thermoplastic polymer coating adds to the overall cost of the process.
Accordingly, it would be desirable to be able to provide a thermally assisted transfer process for transferring dry toner particles having a particle size of less than 8 micrometers from an element to a receiver in which a thermoplastic polymer coated receiver is utilized such that all of the benefits and advantages afforded by the use of a thermoplastic polymer coated receiver in a thermally assisted transfer process are retained, including the transfer of virtually all of the toner particles from the element to the receiver, but one which does not require the use of a coating or layer of a release agent on the thermoplastic polymer coating on the receiver substrate (or the element) in order to prevent the receiver from adhering to the element during transfer and subsequent separation from the element.
In copending U.S. application Ser. No. 455,673, entitled "Thermally Assisted Transfer Of Electrostatographic Toner Particles To A Thermoplastic Bearing Receiver", filed Dec. 22, 1989, it is disclosed that such fine toner particles can be transferred from the surface of an element to a thermoplastic polymer coated receiver with virtually 100% toner transfer efficiency using the thermally assisted method of transfer in the absence of a layer or a coating of a release agent on the thermoplastic polymer coating on the receiver substrate in order to prevent the thermoplastic polymer coating from sticking or adhering to the element surface during transfer of the toner particles from the surface of the element to the thermoplastic polymer coated receiver and during the subsequent separation of the receiver from the element if (i) the surface layer of the element on which the toner particles are carried and from which they are to be transferred to the receiver comprises a film-forming, electrically insulating polyester or polycarbonate thermoplastic polymeric binder resin matrix and has a surface energy of not more than approximately 47 dynes/cm, preferably from about 40 to 45 dynes/cm; (ii) the thermoplastic polymer coating on the receiver substrate to which the very fine, small toner particles are to be transferred comprises a thermoplastic addition polymer which has a glass transition temperature or Tg which is less than approximately 10.degree. C. above the Tg of the toner binder and the surface energy of the thermoplastic polymer coating on the substrate is approximately 38 to 43 dynes/cm, and (iii) the receiver is heated to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate is at least approximately 15.degree. C. above the Tg of the thermoplastic addition polymer during toner transfer and the temperature of the receiver is maintained at a temperature such that the temperature of the thermoplastic polymer coating is above the Tg of the thermoplastic polymer immediately following transfer during the time when the receiver separates from the element. This was a surprising result since it would not be expected that the thermoplastic polymer coating would selectively adhere only to the toner particles during toner transfer without also adhering to the element surface due to the similarities of the surface energies, as expressed in dynes/cm, of the thermoplastic polymer coating and the element surface since it is empirically known that generally, surfaces formed of thermoplastic polymeric materials having similar surface energies tend to adhere or stick to one another when they are brought into intimate contact with one another, as in the situation, for example, where the surface of a toner particle bearing element is brought into intimate contact with and pressed against a thermoplastic polymer coated receiver to effect the transfer of the toner particles from the element surface to the surface of the thermoplastic polymer coating.
In copending U.S. application Ser. No. 455,676, entitled "Thermally Assisted Method Of Transferring Small Electrostatographic Toner Particles To A Thermoplastic Bearing Receiver" filed Dec. 22, 1989, it is disclosed that such fine toner particles can also be transferred from the surface of an element to a thermoplastic polymer coated receiver with virtually 100% toner transfer efficiency using the thermally assisted method of transfer in the absence of a layer or a coating of a release agent on the thermoplastic polymer coating on the receiver substrate when the thermoplastic polymer coating on the receiver substrate is formed of or comprises a thermoplastic condensation polymer, as distinguished from and in contrast to, a thermoplastic addition polymer if (i) the surface layer of the element on which the toner particles are carried and from which they are to be transferred to the receiver comprises a film-forming, electrically insulating polyester or polycarbonate thermoplastic polymeric binder resin matrix and has a surface energy of not more than approximately 47 dynes/cm, preferably from about 40 to 45 dynes/cm; (ii) the thermoplastic polymer coating on the receiver substrate to which the very fine toner particles are to be transferred is a thermoplastic condensation polymer and has a glass transition temperature or Tg which is less than approximately 10.degree. C. above the Tg of the toner binder, and (iii) the receiver is heated to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate is at least approximately 5.degree. C. above the Tg of the thermoplastic polymer during toner transfer and the temperature of the receiver is maintained at a temperature such that the temperature of the thermoplastic polymer coating is above the Tg of the thermoplastic condensation polymer immediately following transfer during the time when the receiver separates from the element. This is an even more surprising discovery because not only would it be unexpected for a thermoplastic polymer coating formed of a thermoplastic condensation polymer to selectively adhere only to such very small, fine toner particles during toner transfer without also adhering to the element surface due to the similarities of the respective surface energies of the thermoplastic polymer coating and the element surface, but also for the additional reason that both the thermoplastic polymer coating and the polymeric binder resin matrix of the surface layer of the element on which the toner particles are carried are composed of thermoplastic condensation polymers which, when pressed into intimate contact with one another during toner transfer, would be expected to adhere or stick to each other as a result of molecular interaction between and bonding of the respective coating and element surface materials upon contact.
Thus, there now exists a means of non-electrostatically transferring very small, fine toner particles having a particle size of less than 8 micrometers from the surface of an element to a receiver using a thermally assisted method of transfer in which a thermoplastic polymer coated receiver can be utilized such that all of the aforementioned benefits and advantages afforded by the use of a thermoplastic polymer coated receiver are retained, including the transfer of virtually all of the toner particles from the element to the receiver, and one which does not require the use of a coating or a layer of a release agent on the thermoplastic polymer coating in order to prevent the receiver from adhering to the element during toner transfer and subsequent separation of the receiver from the element. This achievement constitutes a much needed and long sought after improvement in the thermally assisted transfer process.
However, there is one problem or disadvantage inherent in the process regarding the use of high molecular weight thermoplastic addition polymers or mixtures of such polymers such as those disclosed in previously mentioned copending U.S. application Ser. No. 455,633 entitled "Thermally Assisted Transfer of Electrostatographic Toner Particles To A Thermoplastic Bearing Receiver", filed Dec. 22, 1989, having weight average molecular weights in the range of from approximately 20,000 to 500,000 to form the polymeric coating on the receiver substrates which are used in the process. The problem resides in the fact that when such high molecular weight thermoplastic addition polymers are used to form the polymeric coatings, the receiver must be heated to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate is at least approximately 15.degree. C. above the Tg of the thermoplastic addition polymer or polymers which form the coating in order for the coating material to melt and/or soften or flow sufficiently enough to permit the toner particles to adhere to or to become slightly or partially embedded in the polymer coating during toner transfer so that all or virtually all of the toner can be removed from the element surface during transfer. This is undesirable because ideally it is most advantageous and desirable to heat the receiver to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate is at or just slightly above the Tg of the thermoplastic polymer during toner transfer when removing the toner particles from the element because the higher the temperature to which the thermoplastic polymer coating must be heated above the Tg of the thermoplastic polymeric material which forms the polymer coating in order for the thermoplastic polymer coating to melt and/or flow sufficiently enough to permit the toner particles to adhere to or become slightly or partially embedded in the polymer coating during toner transfer, the greater the tendency of the thermoplastic polymer coating to adhere to the element surface during transfer when it contacts the element. Also, the higher the temperature to which the thermoplastic polymer coating must be heated above the Tg of the thermoplastic polymeric material which forms the polymer coating in order for the thermoplastic polymer coating to melt or soften sufficiently enough for the toner particles to adhere to or become partially embedded in the polymer coating during toner transfer, the greater the tendency of the toner particles to melt and flow or blend together into a localized mass during transfer instead of remaining sintered or fused at localized regions on the individual toner particle surfaces which are in contact with one another during toner transfer and deposition on the polymer coating which is a requirement of the thermally assisted transfer process. Further, the higher the temperature to which the thermoplastic polymer coating must be heated above the Tg of the thermoplastic polymeric material which forms the polymer coating in order for the thermoplastic polymer coating to melt sufficiently enough for the toner particles to adhere to or become partially embedded in the polymer coating during toner transfer, the greater the risk of damage to the element due to a softening of the element caused by the additional heat when the receiver contacts the element during transfer. Still further, the higher the temperature to which the thermoplastic polymer coating must be heated above the Tg of the thermoplastic polymeric material which forms the polymer coating in order for the thermoplastic polymer coating to melt and/or flow sufficiently enough for the toner particles to adhere to or become partially embedded in the polymer coating during toner transfer, the greater the risk of the toner melting and adhering or fusing to the element surface during toner transfer, especially where the coating is heated to a temperature which approaches or exceeds about 25.degree. C. above the Tg of the polymeric material. Finally, the higher the temperature to which the thermoplastic polymer coating must be heated above the Tg of the thermoplastic polymeric material which forms the polymer coating during toner transfer, the greater the risk of blistering the receiver substrate during transfer. Conversely, if the receiver is heated to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate is less than approximately 15.degree. C. above the Tg of the thermoplastic addition polymer or polymers during toner transfer, typically less than 50% and, more typically, less than 10% of the toner particles will transfer from the element surface to the thermoplastic polymer coating during toner transfer.
Accordingly, it would be highly desirable to be able to provide a thermally assisted transfer process for transferring dry toner particles having a particle size of less than 8 micrometers from the surface of an element to a thermoplastic polymer coated receiver in which thermoplastic addition polymers can be utilized to form the thermoplastic polymer coating on the receiver substrate such that all of the previously mentioned benefits and advantages afforded by the use of such a thermoplastic polymer coated receiver can be retained, including the transfer of virtually all of the toner particles from the element to the receiver in the absence of a coating or layer of a release agent on the thermoplastic polymer coating in order to prevent the receiver from adhering to the element during toner transfer and subsequent separation of the receiver from the element, and one in which the temperature of the thermoplastic polymer coating on the receiver substrate does not have to be heated to a temperature that is at least approximately 15.degree. C. above the Tg of the polymeric material which forms the polymer coating during toner transfer in order for the thermoplastic polymer coating to melt and/or flow sufficiently enough to permit the toner particles to adhere to or become partially embedded in the polymer coating during transfer so that all of the aforementioned problems associated with the use of such high transfer temperatures can be avoided.
It should be noted that the use of thermoplastic addition polymers as the polymeric material with which to form the receiver coatings is highly desirable and very important in the thermally assisted transfer process when toner binders are used in the process which are also made of thermoplastic addition polymers because of the increased tendency of the toner binder material to adhere to or become partially or slightly embedded in the polymer coating during toner transfer due to the natural affinity of the materials for each other. The present invention provides such a process.
It has now been found that by using, as the thermoplastic polymer coating materials with which to form the thermoplastic polymer receiver coatings used in the thermally assisted method of transfer, a blend or mixture of certain high and low molecular weight thermoplastic addition polymers, in specific amounts, that toner particles having a particle size of less than 8 micrometers can be transferred from the surface of an element to the thermoplastic polymer coated receiver with virtually 100% toner transfer efficiency using the thermally assisted method of transfer without having to apply a coating or a layer of a release agent to the toner contacting surface of the thermoplastic polymer coating on the receiver thermoplastic polymer coating on the receiver substrate prior to toner transfer in order to prevent the thermoplastic polymer coating from adhering or sticking to the element surface during transfer of the toner particles from the surface of the element to the thermoplastic polymer coated receiver and during the subsequent separation of the receiver from the element and, further, without having to heat the thermoplastic polymer coating on the receiver substrate to a temperature such that the temperature of the thermoplastic polymer coating must be at least approximately 15.degree. C. above the Tg of the thermoplastic addition polymers which form the polymer coating during toner transfer in order for the thermoplastic polymer coating to melt and/or flow sufficiently enough to permit the sintered toner particles to adhere to or become partially embedded in the thermoplastic polymer coating during toner transfer. By utilizing such a blend as the polymeric coating material, it has been found that the thermoplastic polymer coating on the receiver substrate only has to be heated to a temperature such that its temperature during toner transfer is only approximately at least about 5.degree. C. above the Tg of the thermoplastic addition polymers which make up or form the polymeric blend in order for the polymer coating to soften sufficiently enough to allow the toner particles to stick to or become partially embedded in the coating during transfer. This is a significant achievement because now not only can thermoplastic addition polymers be used as the thermoplastic receiver coating materials in the thermally assisted transfer process, but, because the temperature to which the thermoplastic polymer coating must be heated during toner transfer has been reduced, all of the aforementioned problems associated with the use of higher transfer temperatures such as the increased risk of thermal degradation to the element during toner transfer, the increased tendency for the toner particles to melt and flow together into a localized mass during toner transfer, the increased risk of the toner particles adhering to and damaging the element during toner transfer and the increased risk of blistering to the receiver substrate during transfer are avoided.
Specifically, the foregoing achievements and advantages are obtained by blending together to form, as the polymeric receiver coating used in the thermally assisted transfer method, a blend of:
(a) from about 40 to about 90 percent by weight based on the total weight of the blend of a thermoplastic addition polymer having a weight average molecular weight of from about 20,000 to 500,000, a number average molecular weight of from about 5000 to 50,000, and a ratio of weight average molecular weight to number average molecular weight in the range of from about 1:1 to 20:1; and
(b) from about 10 to about 60 percent by weight based on the total weight of the blend of a thermoplastic addition polymer having a weight average molecular weight of from about 1000 to 20,000, a number average molecular weight of from about 500 to 5000, and of ratio of weight average molecular weight to number average molecular weight in the range of from about 1:1 to 10:1;
wherein the Tg of the thermoplastic addition polymers in the blend is less than approximately 10.degree. C. above the Tg of the toner binder and non-electrostatically transferring the dry toner particles which comprise a toner binder and have a particle size of less than 8 micrometers from the surface of an element which has a surface layer comprising a film-forming, electrically insulating polyester or polycarbonate thermoplastic polymeric binder resin matrix and a surface energy of not greater than approximately 47 dynes/cm, preferably from about 40 to 45 dynes/cm, to a receiver which comprises a substrate having a polymeric coating on a surface of the substrate comprising a blend of the aforedescribed thermoplastic addition polymers and where the surface energy of the thermoplastic polymer coating is approximately 38 to 43 dynes/cm and contacting the toner particles with the receiver which is heated to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate is at least approximately 5.degree. C. above the Tg of the thermoplastic addition polymers whereby virtually all of the toner particles are transferred from the surface of the element to the thermoplastic polymer coating on the receiver substrate and the thermoplastic polymer coating is prevented from adhering to the element surface during transfer in the absence of a layer or a coating of a release agent on the thermoplastic polymer coating and separating the receiver from the element while the temperature of the thermoplastic polymer coating is maintained above the Tg of the thermoplastic addition polymers in the blend.